Fix null pointer deref in message_handler.cc

runtime/vm/message_handler.cc

Index: runtime/vm/message_handler.cc
diff --git a/runtime/vm/message_handler.cc b/runtime/vm/message_handler.cc
index 3188675c73d57d68e91455abf6723eb18d6fd837..c92cde06ddd9f135f51262922249edc9dd6cd136 100644
--- a/runtime/vm/message_handler.cc
+++ b/runtime/vm/message_handler.cc
@@ -147,8 +147,8 @@ bool MessageHandler::HandleMessages(bool allow_normal_messages,
   Message::Priority min_priority = (allow_normal_messages && !paused()) ?
       Message::kNormalPriority : Message::kOOBPriority;
   Message* message = DequeueMessage(min_priority);
-  intptr_t message_len = message->len();
   while (message != NULL) {
+    intptr_t message_len = message->len();
     if (FLAG_trace_isolates) {
       OS::Print("[<] Handling message:\n"
                 "\tlen:        %" Pd "\n"